Carolina Wrens in Winter

Several people have talked to me about a new visitor at their bird feeders this winter compared with previous years. The Carolina Wren, a brown bird with a buffy underside and white stripe above the eye, has been frequenting suet feeders and shrubs near houses.

Wrens are insect eaters that occasionally feed on berries and nuts. The limited availability of insects in winter poses a problem for many birds, which generally resolve the problem by either migrating south to where food is available, such as warblers, or shifting their habitat and feeding behavior toward berries or nuts, such as our local wintering bluebirds and titmice. The Carolina Wren has not adopted either strategy and has a rather unusual story.

First, the Carolina Wren is non-migratory. Monogamous pairs tend to set up a territory and defend it all year long. This territory includes nesting habitat, food resources and shelter. The pair defends the territory with a constant vocalization that has been described as "tea-kettle, tea-kettle, tea-kettle."

Second, the Carolina Wren is primarily a southern species. Their range regularly expands north into New England, particularly during mild winters. Some accounts suggest that these are mostly young birds being driven north where they set up resident populations. Then a harsh winter comes along and the local populations are hit hard, sometimes with 100% mortality in a given area, because the food sources on the ground are covered by snow and the birds don’t usually survive prolonged periods when the temperature is below 20°F. The main population to the south survives and the cycle begins again, with new birds dispersing north. This pattern of mortality followed by re-expansion back north has been documented at least twice in the past century—a harsh but amazing tale of survival from a small brown bird in our backyards.

Other resources:

• Birds at Your Feeder by Erica Dunn and Diane Tessaglia-Hymes, 1999.

• The Sibley Guide to Bird Life and Behavior edited by Chris Elphick, 2001.

• BirdSource website: <www.birdsource.com>.